Cathodic sputtering is widely used for the deposition of thin layers of material onto substrates. Basically, a sputtering process requires gas ion bombardment of a target formed from a material that is to be deposited as a thin film or layer on a given substrate. During such a process, the target is positioned such that a sputtering surface of the target faces the substrate across a chamber which has been evacuated and refilled with an inert gas, preferably argon. A high voltage electrical field is applied between the target, which acts as a cathode, and an anode located near the sputter target. The electric field induces electrons drawn from the cathode to collide with the molecules of the inert gas, thereby ionizing the gas. Positively charged gas ions are attracted to the cathode, where the ions dislodge minute quantities of material from the sputtering surface. The dislodged target material traverses the evacuated enclosure and deposits to form the thin film on the substrate.
Sputter target assemblies often are formed through bonding rear surfaces of the targets to backing plates. In addition to providing support for the targets, the backing plates also serve to conduct electrical power to the targets and to dissipate heat generated during sputtering processes. The latter function often is carried out by affixing heat exchange tubing to the backing plate. It generally is desirable that the electrical and thermal properties of the backing plate material be matched to those of the target material so as to minimize impedance as well as the risk of differential expansion between the two materials.
As the sputtering process proceeds, the sputtering surface itself erodes. Generally, this erosion is not uniform across the surface of the target. Often, an applied magnetic field is used to improve the efficiency of the sputtering process. Such magnetic fields tend to induce the formation of erosion grooves along “sputter tracks” on the sputtering surfaces. It frequently is possible to predict the pattern of the sputtering tracks which will form on a particular type of target in a particular type of sputtering system.
As the sputtering process continues, the sputtering tracks deepen, eventually burning through a rear surface of the target to the interface between the target and the backing plate. The burn-through of the interface will be referred to hereinafter as a “target end-of-life condition.” Continued sputtering after the target end-of-life condition is reached leads to the possibility of contamination of the substrate due to sputtering of the backing plate material or, when no backing plate is present, to the possibility of rupture and severe damage to the sputtering chamber due to re-pressurization of the chamber or leakage of heat exchange fluid.
Two competing considerations determine the useful life of a sputter target. On the one hand, since sputtering is often performed using high purity metals or other expensive materials, it is desirable to obtain as much use from a target as possible. On the other hand, it is undesirable to continue use of a target after a target end-of-life condition is reached. Although it is known to monitor target life by maintaining a count of the number of sputtering cycles in which a particular target has been used, it is possible for the counter to be reset or for the user to be overly aggressive in target consumption. Hence, there remains a need in the art for methods for detecting target end-of-life conditions.
It is known to provide visual means for detecting target end-of-life conditions. One known technique involves painting colored dots or other patterns on the backing plate along the sputtering track so as to enhance the operator's ability to visually identify a target end-of-life condition. Another known technique uses a backing plate material visually distinguishable from the target material, so that the operator will readily identify deposits of sputtered backing plate material on the substrate. Each of these techniques has the drawback that the detection of a target end-of-life condition is dependent on the diligence of the operator throughout the sputtering process. The use of a backing plate material visually distinguishable from the target material has the additional drawback that visually distinguishable materials also likely will have different electrical and thermal properties, thereby increasing the impedance as well as the risk of differential expansion between the two materials.
As presently advised, it is also known to form sputter target assemblies in which pockets of pressurized gas are formed at the interface between the target and backing plate along the sputter tracks. When a target end-of-life condition occurs, these pockets of pressurized gas are ruptured, thereby increasing the pressure within the sputtering chamber. One drawback to this technique is the difficulty of forming such pockets of pressurized gas during the fabrication of the sputter target assembly. Another drawback is that the thermal conductivity of the pressure gas will likely be less than that of the surrounding target and backing plate materials, thereby risking thermal damage to the sputter target assembly.
During a sputtering process, the material removed from the sputtering surface forms a plasma in the region between the target and the substrate. Where dielectric impurities such as oxides or other ceramics are present along the sputtering surface, small electrical arcs tend to appear on the surface of the target. In the past, the occurrence of such arcs have been considered undesirable, since they tend to cause localized heating of the sputter surface. This localized heating increases the risk of macroparticle ejection, that is, the ejection of unusually large particles or droplets of sputtering material deleterious to the uniformity of the thin film of sputtering material formed on the substrate.
Therefore, there remains a need in the art for methods and apparatus for detecting target end-of-life conditions without operator intervention. There is a further need for sputter target assemblies designed to facilitate automatic target end-of-life conditions.